Electromagnetic Compatibility is a crucial necessity for electronic goods marketed in most global markets. The majority of EMC design factors are considered during the phase of Printed Circuit Board (PCB) design. Therefore, this article will explore various common strategies and techniques that can enhance the performance of a PCB.
Definition of EMC
Electromagnetic Compatibility (EMC) is all about designing electronic circuits in a way that minimizes their impact on other electronic devices and helps them resist interference from other circuits. Even though most circuits can work without specifically considering EMC, many countries have rules that demand commercial products meet strict EMC standards. To tackle EMC, it’s essential to understand electromagnetic emissions—these are stray radio waves produced by alternating or switching currents. Designers aim to prevent conductors from acting like strong antennas that emit these radio waves.
EMC emissions can be understood as unwanted radio waves that are produced by alternating or switching currents. In this scenario, any conductor carrying an alternating current can function as an antenna. Therefore, the responsibility of a designer is to ensure that all conductors in a product are poorly suited as antennas, thus preventing them from emitting potent radio waves.
Preliminary design factors
Electromagnetic emissions, being radio waves, are commonly produced by high-speed switching devices, data buses, and radio circuits. Switch-mode power supplies are infamous for creating EMC problems and introducing switching noise into power lines, while Wi-Fi modules can also pose significant challenges. High-speed data buses like SPI, PCIe, and memory lanes can emit these waves, so their traces need to be meticulously routed to reduce emissions.
Before the traces are laid out on the PCB, the design and layout of the PCB must be thoughtfully planned. While a wealth of information can be found online, it’s essential to adhere to the following principles:
- Components should be arranged according to their function (e.g., analogue, digital, etc.)
- If feasible, connectors should be located on one side of the PCB.
- The distance between interconnected components should be kept to a minimum.
- High-speed connectors should be grouped together and isolated from analogue connectors.
Tips for PCB routing
The process of laying out paths on a Printed Circuit Board (PCB) is as vital as deciding the locations of components and connectors. Similar to the initial design phase, there are numerous factors to consider when routing traces. There are plenty of freely available examples of good Electromagnetic Compatibility (EMC) practices online, but the information provided here is just a glimpse of the possibilities.
Layers of PCB Two-layer PCBs are often the most cost-effective choice, but they lack the ability to utilize power planes. PCBs with more than two layers can dedicate an entire layer to a power rail (like 5V) or ground, known as power planes. For a four-layer PCB, it’s optimal to use the two inner layers as power planes. This arrangement maximizes the capacitance between the layers and minimizes return paths for current loops and emitted emissions.
Pairs of Differential Signals Differential pairs, commonly found in USB, should always be routed side by side without any traces or planes in between. It’s vital that these traces are of equal length for impedance matching. If the traces are too far apart or have unequal impedance, it can affect the differential pair’s performance (i.e., noise and speed) and can create current loops leading to radio emissions.
Protective Traces Guard traces are specially routed ground traces that flank a trace on either side. They can provide immunity from external sources and prevent stray emissions from the trace from escaping (acting as a simple Faraday cage that encloses the trace).
Via Stitching Stitching vias are often connected to ground and encircle particularly noisy areas. They are common on PCBs and, like guard traces, act as a Faraday cage to help absorb stray EM emissions. The spacing between stitching vias should generally be below λ/20, where λ represents the maximum frequency being guarded against.
For instance, if a 2.4GHz circuit was being guarded, the minimum distance between vias should be 15cm / 20 = 7.5mm. However, such small distances may not always be achievable, and the inclusion of vias generally improves performance regardless. The hole size of the stitching via is generally not a concern, but it’s ideal to use the smallest drill hole offered by the PCB manufacturer that doesn’t incur additional costs.
Minimizing Trace Length To reduce EM emissions, it’s important to keep trace lengths as short as possible, as this essentially makes them a poor antenna. However, it’s also crucial to ensure that the trace length isn’t a multiple or even a fraction of the wavelength of the expected nominal frequency of the signal that will be present in that trace.
For example, if a 2.4GHz signal is to be present in a trace, and it’s not a radio link (i.e., a standard data bus lane), then the length of that trace should not be a multiple or even a fraction of the wavelength, which is 15cm. Therefore, trace lengths of 15cm, 7.5cm, and 5cm should all be avoided, and traces over 15cm should not be used at all.
Summary
The strategies discussed in this article represent just a handful of ways to mitigate Electromagnetic Compatibility (EMC) issues. Delving deeper into freely available online resources will uncover the intricate nature of EMC design. However, the most effective approach to minimize EMC is to comprehend its root causes and eliminate these sources prior to implementing methods to reduce EMC by trapping stray emissions (for instance, using Faraday cages).